Free piston machine operation



Sept. 2, 1958 2 Sheets-Sheet 1 Filed Oct. 4, 1955 8 mn R S w M N m E. m3,6331 32:20 V L. T N M m 2:32 WL I M B A f s m N v ON N\\.\ ml N cw ARIN I In 2 H I mu m. m I FN\ Sept. 2, 1958 F. M. LEWIS FREE PISTONMACHINE OPERATION 2 Sheets-Sheet 2 Filed 001;. 4, 1955 RS/MW 0 W MMMMA AR F V. B m Z 0 I 7 F 4 Unite States Patet FREE PrsToN MACHINE OPERATIONFrank M. Lewis, Weston, Mass.,- assignor, by mesne assignments, to theUnited States of America as represented by the Secretary of the NavyApplication October 4, 1955, SerialNo. 538,546

1 Claim. (Cl. 123--46) This invention relates to internal combustionmachines such as air compressors and gas generators of the free pistontype and in particular is concerned with a system or apparatus foroperating a group of such machines in synchronism.

Free piston machines, as is generally known, include a motor piston anda compressor piston which operate in unison and function to supply airor gas for the operation of other machines such as turbines. A freepiston machine as distinguished from other piston machines operatesother equipment directly by the developed power rather than throughmechanical connections.

Compressed air developed by the compressor portion of a free pistonmachine is usedto scavenge and charge the motor portion of the machineand the motor portion delivers the developed power gas which constitutesa mixture of combustion products and scavenging air to operate otherequipment, for example, to drive a turbine.

When a group of two or more free piston gas generators are used tooperate a turbine, it is advantageous that they be operated insynchronism. Such synchronous operation requires that all of. the freepiston machines operate at exactly the same cyclic speed, and that theirrespective piston positions he in predetermined phase relationship witheach other. Such an operation is particularly desirable where a group ofsuch gas generators has a common air intake manifold or a common exhaustor delivery manifold. Synchronized operation provides a substantiallyuniform pressure within the gas generating system. Also, by operating agroup of gas generators in synchronism, the operating noises should tendto be reduced to a minimum.

An object of this invention is to provide apparatus for controlling theoperation of free piston machines in accordance with pressureconditions'iir: the machine.

Another object is to provide. an arrangement of control elements whichsimplifies the synchronous operation. of a plurality of free piston gasgenerators.

Still another object of the invention is to utilize inexpensive andreadily available control element-s for synchronizing the operation of aplurality of free piston gas generators.

Other objects and advantages of the invention will hereinafter becomemore fully apparent from the following description of the annexeddrawings, which illustrate a preferred embodiment, and wherein:

Fig. l is a sectional view of a free piston type gas generator withcontrol elements and fluid lines shown in elevation; and

Fig. 2 is a generally diagrammatic view of a plurality of suchgenerators arranged for operation in synchronism.

In Fig. l of the drawing is indicated generally a free .piston machineof well known structure comprising a casing containing two opposed andidentical, diesel type operated, air compressing units 11 and 12. Eachof the air compressing units comprises a power piston V 13 and anintegrally connected air compression piston 14.

On the opposite side of each piston 14 is provided a direct bouncepiston 15. The two power pistons 13 cooperate to form, with the casing10, and generally centrally thereof, a power chamber 16 into which fuelis injected, for example, by way of a nozzle 17 for compression andignition when the'power pistons 13 attain a predetermined inner positionas indicated in Fig. 1. The power developed in chamber 16 drives thepiston assemblies of both units outwardly to effect the air compressionstroke. As the power pistons 13 move outwardly after the ignitionstroke, they will uncover an exhaust or outlet conduit 18 for escape 'ofburnt gases from the ignition chamber 16 and a scavenger conduit 19 toallow entry of air to chamber 16 for scavenging purposes and passagetherefrom and into conduit 18 from which the gases are directed foruseful work. Air is admitted through conduit 19 prior to the compressionand ignition step of the cycle along with the supply of fuel throughnozzle 17. The piston assemblies are arranged to move equally andoppositely by the usual mechanical connections (not shown).

At the power piston side of each air compression piston 14- is a chamberknown as an inner or reverse bounce chamber Zll and on the opposite sideof each of these pistons i4 is a compression chamber 21. Each of thecompression chambers 21 is provided with intake valves 22 and exhaustvalves 23.

The direct bounce pistons 15 form, at opposite ends of casing 10, directbounce chambers 24 in which air will be compressed on the outwardmovement of the piston assemblies to provide a cushion for damping orstopping the outward movement of the piston assemblies and also toprovide for the return or inward movement of the piston assemblies toeffect the compression-ignition step of the cycle of operation.

The direct bounce chambers 24 are in communication with each otherthrough an equalizing line 25, and the reverse bounce chambers 20 are incommunication with each other through a second equalizing line 26. Afterthe initial starting of the machine, the piston assemblies willreciprocate under the pressures existing in the various chambers andprovide the desired cycle of operation during which the air passingthrough compression chambers 21 from the inlet valves 22 will, throughvalves 23, enter the scavenging zone 27 from which it passes to theignition chamber 16 through conduit 19 and the scavenge gases exhaustedthrough conduit 18 for the desired use.

The apparatus for effecting speed control of the free piston machine isshown in Fig. 1 and is arranged to add to or remove air from the directbounce chambers 24. This is accomplished in accordance with pressureexisting in the free piston machine system. Extraneous control airsupplied to the direct bounce chambers 24 will increase the cushioningor compressing eifect of these chambers and hence speed up operation ofthe piston assemblies. Conversely, control air supplied to the reversebounce chambers 20 through line as will decrease the speed of operationof the pistons.

Control air can be supplied by the lines 25 and 26 simultaneously to thedirect bounce and reverse bounce chambers, in such ratio that the speedof the piston assemblies may be increased or decreased as desired.

In Fig. l, the apparatus for controlling the speed of the pistonassemblies is shown connected with the equalizing line 25 which is incommunication With the direct bounce chambers 24. A control valve 30 ispositioned in branch line 31, of the equalizing line 25, and is providedwith a rotary valve plug 32 which is incommunication With a main airsupply line 34. The rotary valve plug 32 is so designed as to pass onlya small amount of air from the main conduit 34 to the branch conduit 31during each of its revolutions. The main air conduit 34 is provided withan air chamber 35, a pressure regulator 36 and therebetween a throttlevalve 37. Air from a source is supplied by line 38 to the regulator 36and ex cess air may be discharged through line 39.

The operation of the rotary valve 32 is effected, as shown, by means ofelectric motor M the operation of which is controlled by a rheostat 40in circuit with a current source and the motor. The position of the arm(not shown) of rheostat 41) is controlled by a diaphragm pressureregulator 41 which is in turn actuated by the pressure in the exhaustconduit 18. The diaphragm regulator 41 may be located in other pressurelines or chambers of the system such as chamber 27, or located in thefuel supply line, turbine lines, etc. A bypass line 45 for the valve 30is disposed between main air line 34 and branch line 31 and has a valve46 therein, and another valve 47 is provided between the air chamber 35and valve 30.

These valves are used in starting the free piston machine which isaccomplished by closing valve 47 and opening valve 46 to supply airdirectly to the equalizing line 25 from the air chamber 35 and thence tothe direct bounce chambers 24 which moves the piston assemblies inwardlyand the compression of air in chamber 16 with the fuel supplied theretowill effect ignition in chamber 16. During this starting operation airpressure in the direct bounce chambers 24 is controlled by the airpressure regulator 36 which is in communication with the exhaust line 18through line 48.

After the free piston machine is set in operation, the motor M operatesthrough regulator 41 and since it is controlled according to thepressure in the system, the motor will have substantially the samecyclic speed as the free piston machine. Valve 46 is then closed andvalve 47 is opened so that air from the main air line 34 is supplied tothe equalizing line 25 under control of rotary valve plug 32. Thiscontrol system provides an arrangement for operating the free pistonmachine and the valve 30 in synchronism and although there may be sometendency of the cyclic speeds of the motor or the free piston machine tohunt, this hunting tendency can be minimized by varying the size of theair chamber 35 or by adjusting the setting of the valve 37.

Fig. 2 shows a plurality of free piston machines similar to the freepiston machine shown in Fig. 1, each of which is provided with thecontrol arrangement of Fig. 1 and arranged to operate in synchronism.Since the control system described in connection with Fig. 1 willsynchronize the operation of a free piston machine with the electricmotor M, a plurality of such machines can be held in synchronism withone motor. Thus, the pressure existing in one machine, which would beconsidered the master, can control a single motor and the motor arrangedto regulate the supply of control air to the reverse or direct bouncechambers of each free piston machine and provide synchronous operationof the machines. The driving connections between the motor con- '4trolled by the master machine and the valves of the other machines ofthe group can be mechanical as shown schematically in Fig. 2 or thedrive may be electrically operated through synchronous or Selsyn motor.

Operation of the motor M will be considered to be controlled by themachine shown in Fig. 1, which will be considered the master machine.The free piston machines shown diagrammatically in Fig. 2 will beconsidered the slave machines and controlled in accordance withoperation of the motor M. The valves 30 of the machines in Fig. 2 willbe considered to be mechanically connected in a manner that all areoperated through the motor M. Exhaust products from each machine isdirected through its exhaust or outlet conduit 18 to a manifold 50 fromwhich it is supplied to a turbine or machine which can be operateddirectly by air.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

What is claimed is:

In a free piston engine comprising a casing having opposed pistonassemblies arranged for reciprocation therein and providing a variablevolume central combustion chamber, a pressure system including a pair ofouter bounce chambers interconnected by a first pressure equalizingline, a pair of inner bounce chambers interconnected by a secondpressure equalizing line, a first compression chamber between one of theouter bounce and inner bounce chambers, a second compression chamberbetween the other outer bounce and inner bounce chamber, a scavengeconduit providing communication between the first and second compressionchambers and the combustion chamber and an exhaust manifold incommunication with the combustion chamber, the improved means forregulating the reciprocation of the piston assemblies which comprises anair supply source, a main air conduit and a branch conduit between saidair source and the first pressure equalizing line, a motor operatedvalve between the main and branch conduits, a pressure device incommunication with the exhaust manifold for actuating said motoroperated valve in accordance with exhaust pressure, a pressure regulatorin the main conduit between the motor operated valve and the air supply,a third conduit between the pressure regulator and the exhaust manifoldand a by-pass conduit for the valve between the main and branchconduits.

References Cited in the file of this patent UNITED STATES PATENTS2,434,280 Morain Ian. 13, 1948 2,435,970 Lewis Feb. 17, 1948 2,659,194Huber Nov. 17, 1953 FOREIGN PATENTS 595,117 Great Britain Nov. 27, 1947

